A known technique for obtaining a resin material which has high strength and in which a brittle fracture tends to be suppressed is to reinforce a resin with carbon fibers to obtain a composite material. In particular, a composite material obtained by reinforcing a thermoplastic resin as a matrix resin with carbon fibers (the composite material are also called a carbon-fiber-reinforced thermoplastic resin, and are hereinafter often abbreviated to CFRTP) is excellent in terms of processability and suitability for recyclability and are expected to be utilized in various fields.
As a method for manufacturing a composite material including carbon fibers and a thermoplastic resin, a method in which carbon fiber bundles are impregnated with a molten-state thermoplastic resin having a relatively high viscosity to obtain a composite material. In this manufacturing method, it is necessary that an impregnation treatment for carbon fiber bundles with a thermoplastic resin is conducted for a long term at an excessively high pressure under a condition where a melt viscosity of the thermoplastic resin is lowered by elevating an ambient temperature, in order to avoid a trouble that strength of a molded article is reduced due to insufficient impregnation. In such an impregnation treatment for a long term at a high pressure, there has been a problem, for example, that manufacturing cost is increased.
As techniques for facilitating the impregnation of a thermoplastic resin into carbon fiber bundles, a method in which carbon fibers are intertwined with thermoplastic resin fibers to prepare fiber bundles, and heat and pressure are applied thereto to melt the thermoplastic resin so as to facilitate impregnation (patent document 1), a method in which carbon fiber bundles are impregnated with a molten resin having a low molecular weight and then impregnated with a thermoplastic resin having a high molecular weight (patent document 2), and a method in which carbon fibers are opened in a bath of a molten resin and are impregnated therewith (patent document 3), and the like, are disclosed. Furthermore, patent document 4 describes a method for facilitating impregnation of a thermoplastic resin into carbon fibers themselves, the method for adjusting a sizing agent of the carbon fibers to be a sizing agent having good wettability with the resin.
As described above, in the conventional production methods, it has been necessary to conduct an independent impregnation step under special conditions therein in order to sufficiently impregnate reinforcing fiber bundles with a thermoplastic resin. Because of this, the conventional methods are still unable to provide CFRTP products (shaped products) with excellent properties and appearance at such a low manufacturing cost that these products are increasingly usable in various applications, and do not sufficiently meet expectations of each field for CFRTP. Particularly, for CFRTP in which a thermoplastic polyamide (hereinafter sometimes abbreviated as PA) is used as the thermoplastic resin, it is expected to be applicable to various functional parts such as automobile parts and prompt solution of the above problem has been desired.